The use of zinc containing coatings to prevent corrosion of steel substrates is well known. The zinc and the metal substrate set up an electrochemical reaction in which the zinc particles are reduced, acting as electron donors. For this reason these types of coatings are often called cathodic coatings. Traditionally, it has been taught that very high concentrations of zinc particles must be used in order for the ions flow freely. This teaching has limited attempts to use fillers in order to lower the cost by reducing the amount of zinc present.
In U.S. Pat. No. 4,891,394 issued to Ronald R. Savin, an attempt was made to lower the percentage of zinc in a coating by replacing a portion of the zinc with hollow ceramic spheres. The spheres are lightweight and economical, but would theoretically reduce conductivity in the coating, as the spheres would separate the zinc particles from one another. For this reason, the spheres were coated with zinc prior to being incorporated into the coating in order to enhance conductivity.
U.S. Pat. No. 5,252,632 issued to Savin discloses the use of uncoated hollow glass spheres as filler. By using lightweight spheres Savin '632 teaches that a significant amount of filler can be added while maintaining the necessary conductivity. Savin '632 discloses the use of microspheres, specifically microspheres having diameters up to 150 microns.
The polymers disclosed in the patent are defined in three groups. The first group includes epoxy resins, vinyl chloride resins copolymerized with isocyanates, polyurethane resins, polyester resins, oil-modified polyester resins or alkyds, polymers of acrylic and methacrylate esters and mixtures thereof. The second group includes lattices prepared by the emulsion polymerization of monomers selected from the group consisting of acrylic, methacrylic or vinyl monomers, water-borne urethanes and mixtures thereof. The third group includes polymers which are suited for use in powder coatings applied by electrostatic means or by fluidized bed immersion techniques, including polyester powders, epoxy powders, acrylic powders, and urethane powders.
Disadvantages of the Savin '632 coating include that the addition of the spheres results in pitting of the coating surface as well as splitting or cracking of the coating, known as mud-cracking, which occurs as the solvent evaporates and the coating dries. These cracks can result in loss of anti-corrosion protection in these areas. Mud cracking increases significantly with the thickness of the coating film.